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U.S. Dep. of Agriculture—Agricultural Research Service (USDA-ARS), P.O. Box 459, Mandan, ND 58554
* Corresponding author (merrills{at}mandan.ars.usda.gov)
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ANOVA, analysis of variance • CV, coefficient of variation • GRLD, greatest root length density, defined as the average RLD value in an 0.3-m depth segment with the greatest values in the uppermost 1.2-m of the soil profile • LTA, long-term average (precipitation) • P-wall MR, pressurized-wall minirhizotron • RDI, Rooting Depth Index • RLD, root length density • Stand MR, standard minirhizotron • TRL, total root length
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In dryland agriculture, the replacement of wheatfallow dominated rotations with more complex crop rotations having a diversity of crops is essential for improving soil health and decreasing the depredations of disease, weeds, and insects. Furthermore, cultivation of crops alternative to wheat appears to improve economic survival of farm operators beset by input costs and market dynamics over which they have no control in the current political-economic system. Comparative root growth information will help guide design and implementation of new diverse cropping systems.
Root growth of a plant species has a potential pattern set by genetics, but there is a variable plastic component of root growth that responds to conditions of the soil environment and climatic demands placed on the whole plant (Zobel, 1992). Brouwer and deWit (1969) emphasize the dynamic balance between root and aboveground growth for plant growth modeling: deficiencies in water and nutrient supply can result in greater belowground allocation of physiological resources and increased root growth. Different patterns of root growth responses to water regimes in semiarid zones are reported in the literature. In one pattern, more frequent irrigation or rainfall results in more shallow depths of root growth (Hoogenboom et al., 1987, with soybean; Merrill and Rawlins, 1979, with sorghum [Sorghum bicolor (L.) Moench]). Another response is the tendency for root growth to penetrate less deeply in the profile as drought lowers the water content of subsoil (Merrill et al., 1996, with spring wheat).
Root growth systems are hierarchical, and each lower-ordered class of root members has lower diameter, less mass per length unit, shorter length, and less life span than higher-ordered roots. Thus, much of a plant's root length is found in the finer and smaller members of root systems. For example, Fiscus (1981) reported that two-thirds of the total area of Phaseolus vulgaris L. roots had an average diameter of 0.2 mm while about one-fifth of total area was on roots averaging 0.5-mm diameter. Zobel (1992) summarizes two studies on corn (Zea mays L.) and one on barley (Hordeum vulgare L.) as showing that the smallest class of roots have an active growing period of 
2 d with a lifetime of 2 wk.
With the exception of very careful and time-consuming floatation-pail techniques, the majority of root recovery methods do not assure the conservation of the finer more active and more fragile part of root systems. The use of clear plastic tubes, known as minirhizotrons, installed in the field (Taylor, 1987) has made effective nondestructive measurements of fine root growth possible. One of the most functional versions of this methodology makes use of a miniaturized video camera (microvideo system) that allows both viewing and recording of root growth observations (Upchurch and Ritchie, 1984). The magnification provided by the minirhizotron-microvideo system allows the fine-root fraction to be imaged and measured.
The objective of research presented here was to determine root-length growth of eight crop species (safflower, sunflower, spring wheat, crambe, canola, soybean, dry pea, and dry bean) grown in dryland cropping systems in the Northern Great Plains. The measurements were repeated over a 3-yr period to enable the effect of variant climate on root growth to be assessed.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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0.6-m depth consist of aeolian-derived material. A transition zone, typically 0.2 m or more thick, and consisting of variably coarser-textured material with lime inclusions, underlies this. The glacial-till subsoil is root penetrable and of finer-textured material. The experiment was conducted over a 3-yr period, from 1995 to 1997, and was moved to a new site each year. Alternate crops were in a 3-yr rotation consisting of spring wheat– winter wheat-alternative crop. In 1995, crop plots were split into no-till and minimum-till zones. The 1995 root growth observations were conducted under minimal-till management, which consisted of two passes of an undercutter with 0.81-m sweeps and with application of Sonalan (ethyltrifluralin, C13H14F3N3O4) herbicide at 1.1 kg ha-1 a.i. In 1996 and 1997, a single tillage treatment was applied consisting of a single undercutter pass with Sonalan application. Nitrogen (67.3 kg ha-1) and P (11.2 kg ha-1) fertilizers were applied at time of seeding. Seven crops were seeded in randomized order within three replications using a no-till drill: dry bean, soybean, dry pea, crambe, canola, sunflower, and safflower. Individual plots were 9-m wide by 46-m long. In addition to the seven alternative crops, the root growth of spring wheat growing on land fallowed the previous season was observed for comparison. Because of the previous year of fallowing, the spring wheat crop would usually have greater soil water availability than the alternative crops.
Root Growth Measurements
Two types of minirhizotrons were used: a standard type with rigid walls and a pressurized-wall type. The pressurized-wall minirhizotrons (P-wall MRs) (Merrill, 1992) have a working diam. of 9.6 cm and consist of a rigid inner plastic tube of 7.6 cm-diam. and an outer, flexible, tubular wall of 0.5-mm thick polyvinyl sheeting sealed to the inner tube at either end. After placement in access holes, P-wall MRs were inflated and kept under a constant pressure of 20 kPa using solar panel-powered air pumps. The standard minirhizotrons (stand MRs) were made of Lexan plastic, 5.6-cm outside diam., and 5.0-cm i.d. The P-wall MRs were either 2- or 3-m long, while the standard type were 2-m long. In 1995 and 1996, four minirhizotrons were installed in each of two replications of each crop. In 1997, six minirhizotrons were installed in each of two replications. The mix of minirhizotron types was as follows: In 1995, each crop treatment received six P-wall MRs and two stand MRs. In 1996, six pressurized-wall and two standard types were installed in safflower and sunflower crops, and four of each type were installed in the six other crops. In 1997, safflower and sunflower crops received six of each type, and the other crops received four of the pressurized-wall and eight of the standard type. All P-wall MRs in safflower and sunflower crops were 3-m long, while all minirhizotrons of both types in other crops were 2-m long.
Two different types of minirhizotrons were used because they have apparently complementary working qualities. Standard minirhizotrons have superior viewing quality and P-wall MRs are designed to give physical control of the soil-wall interface.
Access holes for minirhizotrons were drilled by rotary auger using a special tractor-mounted hydraulic soil-sampling probe that could be positioned accurately in the x-y plane and set to an angle. Holes were drilled at an angle of 30° with respect to the vertical. Holes for stand MRs were 5-cm diam., and these tubes were forced into the soil with an hydraulic probe, establishing tight soil-wall interfaces. Holes for P-walls MRs were 10-cm diam., allowing for relatively easy insertion of the tubes into access holes, but contact with the soil was firm when the minirhizotrons were pressurized. Minirhizotrons could not be removed from the soil while under pressurization. Installation of minirhizotrons in a crop occurred within 2 wk or less time after seeding.
Minirhizotrons were viewed with a microvideo camera (Bartz Technology Corp.1, Santa Barbara, CA) at weekly or biweekly intervals. Equipment was mounted on a hand-push field cart, and included a video monitor, higher quality video recorder, and electric generator. Video images were recorded with audio annotation. Minirhizotrons were viewed at two nearly side-by-side places on the upward side at marks placed every 5 cm along the tubes (4.3-cm intervals of depth). The equipment gave 16-fold magnification with 12 by 17 mm viewing areas in stand MRs and 11-fold magnification with 18 by 25 mm viewing areas in P-wall MRs.
When installed in access holes at an angle of 30°, the 2-m minirhizotrons allowed observations to a depth of 1.4 m, and the 3-m minirhizotrons allowed observations to 1.8 m depth. Before the harvesting of crops with machinery, minirhizotrons were extracted from access holes for reuse in subsequent years.
To assure that root growth observations were not confounded by appearance of roots from weed species, areas around and particularly above minirhizotrons were kept weed-free by pulling them out by hand.
Conversion of Minirhizotron Data to Root-Length Density
Minirhizotron images were displayed on a video monitor of 500 cm2 area with three horizontal and three vertical lines superimposed on the screen. Intersections of root images with the monitor lines were enumerated. Only those roots of white or lighter color were counted; dark, brown-colored roots, often partially or fully shrunken, were assumed dead or decayed, and were not counted. Theories for converting minirhizotron observations to equivalent root-length density (RLD) values require root numbers per unit of wall area data (Upchurch, 1987). Root number data consists of a counting of each root member, with a branch being counted as a separate member. Root intersection values were converted to root number data by factors determined through field-made calibrations of intersections vs. numbers using zero-intercept linear regressions.
Root length density (in km m-3) is the most useful measure of root length growth for application to environmental soil science. Minirhizotron root numbers per area data were converted to equivalent RLD values by application of a specific conversion model (Merrill and Upchurch, 1994; Upchurch, 1985). The model is based on an analytical geometric construction that considers root length growth that would occur inside the volume occupied by a minirhizotron if the device were not present in the soil. All possible root growth directions are considered equiprobable, and the model calculates the mean theoretical nonbranched length of root that would grow from a root impinging on the minirhizotron wall. Merrill et al. (1994b) present experimental evidence for the existence of a wide range in the directions of fine-root growth, including directions more upward than horizontal.
The conversion model has been validated by application of studies in which both minirhizotron root number data were taken and direct measurements of RLD were made on root material recovered from soil (Merrill and Upchurch, 1994). To make these comparisons, minirhizotron RLD values were calculated by application of the conversion model. In studies by Meyer and Barrs (1985) in wheat using minirhizotrons of two sizes, regression values linking soil sample-derived RLD to minirhizotron RLD were within 10% of the model-predicted conversion factor. For a study of cotton (Gossypium hirsutum L.; Upchurch, 1985), the regression value between soil sample RLD and minirhizotron RLD was within 7% of the model conversion factor. The regression between RLD from an apparently efficient horizontal minirhizotron installation and soil sample RLD for cotton (Bland and Dugas, 1988) had 50% less value than the model conversion factor. In contrast, regression between RLD of an inefficient mirror-telescope and minirhizotron system and well-recovered soil RLD (Merrill et al., 1987) was three times greater than the model conversion factor.
The actual form of the model conversion equation (Merrill and Upchurch, 1994) is:
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It should be noted that the Merrill and Upchurch (1994) theory for converting minirhizotron root numbers to RLD is somewhat related to the Lang and Melhuish (1970) theory for converting core-break root counts per area to RLD. The Lang and Melhuish theory relates roots intersecting an unit of area to the associated length of roots in a contingent cube of unit volume, and has a conversion factor value of 2.0.
Data Analysis and Display
Because the spring wheat measurements were made on crops growing on fallowed land and all other crops were part of continuously cropped rotations, minirhizotrons in this crop were read on fewer dates and spring wheat data is included only in summary tabularizations of rooting depth measures and greatest RLD (GRLD) and in only one of the figures.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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10% above the LTA. Precipitation during the four months in 1997 was 75% of the LTA, with the two month period of May and June being significantly drier than average, only 45% of the LTA.
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Many examples of minirhizotron-observed RLD profiles in the literature, and indeed, in the present study, exhibit relatively lower values near the soil surface. This has been attributed to such artifacts of the system as poor soil-wall contact at the shallowest depths nearest the soil surface and an inhibition of root growth caused by incomplete shielding from sunlight near the surface (Levan et al., 1987). However, in our study, the earliest RLD profiles for canola and for the three legume crops in 1997 show no lesser values at the shallowest soil depth, 0.04 m, than at immediately greater depths.
The largest RLD values for various crops and years (Fig. 1) show a considerable range of variation, from about 10 km m-3 to somewhat over 60 km m-3. The overall accuracy of RLD values depends on the quality of minirhizotron functioning, including root growth in the wall-soil interface zone and observational efficiency. Accuracy also depends on the quantitative validity of the conversion of minirhizotron root counts to equivalent RLD. To compare our RLD measurements with those found in the literature, we have defined a GRLD value (see Materials and Methods).
For sunflower, our GRLD values (Table 3) range from 6.0 km m-3 for P-wall MRs in 1997 to 26.3 km m-3 for Stand MRs in 1997. Maertens and Bosc (1981) measured RLD from soil cores in field-grown sunflower, yielding a GRLD value of 21.8 km m-3. Jaafar et al. (1993) observed sunflower root growth in the field with the core-break technique, and their root counts may be converted to RLD by the theory of Lang and Melhuish (1970). From their data for two different years, the GRLD values extracted were 3.7 and 7.9 km m-3.
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For soybean, our GRLD values (Table 3) ranged from 8.6 km m-3 for P-wall MRs in 1995 to 29.8 km m-3 for Stand MR in 1997. Mason et al. (1982) measured RLD on material recovered from soil blocks taken from supplementally irrigated soybeans, giving largest two GRLD values of 4.2 and 4.6 km m-3. Nickel et al. (1995) reported soil-core RLD from a field experiment in Minnesota, giving largest two GRLD values of 17 and 25 km m-3.
Fine-root growth tends to be clumped, and minirhizotron root count data often contain many zeroes, tend to be nonnormally distributed, and typically display relatively high coefficient of variation (CV) values. Average annual CV values by minirhizotron type for data used to calculate GRLD values (Table 3) ranged from 50 to 71% for nonzero data and from 87 to 117% for data with zeroes included. Vos and Groenwold (1987) studied minirhizotrons in soil containers and published data showing that CV values of 100% at an RLD of 10 km m-3 declined to 50% as RLD increased to 35 km m-3. Meyer and Barrs (1985) measured root counts at intervals along horizontal minirhizotrons and showed lesser to similar variability for undisturbed large soil cores and disturbed soil: CV values ranging from 33 to 58% at an RLD level of 15 km m-3.
Soil Depths of Root Growth
The time courses of maximum observed root depths and of median depths of root length are shown in Fig. 2
. The time courses of maximum growth depths were not generally as stable as those for median values. Depths of medians were approximately one-half of the maximum depths. The ratios of median to maximum depths (Table 4) ranged from 0.47 for dry bean to 0.57 for spring wheat, and the average ratio value was 0.51.
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As a means of summarizing the rooting depth results, we have constructed a rooting depth index (RDI) that gives approximately equal weight to both the maximum and the median depths.
Because the average median value is roughly one-half of the maximum, multiplying the median by two achieves this:
 | [2] |
Thus, RDI = 2 for a plant with a maximum rooting depth of 2 units and a median depth of 1 unit. By this index, the crops fall into agronomic and botanical family groups (Table 4). Foremost, at RDI = 1.73 is the deeply rooted, dry-loving safflower crop, which is a member of the Compositae family, followed by another oilseed crop, sunflower (RDI = 1.45), also a member of the Compositae family. Spring wheat (RDI = 1.31) is sandwiched between the more deeply rooted oilseeds and the mustard family crops, crambe and canola, with RDI values of 1.17 and 1.13, respectively. The three legume pulse crops, soybean, dry pea, and dry bean are the shallowest rooted, with RDI values of 1.03, 0.97, and 0.96, respectively.
Summarization of root growth data by parameterization and summary equations is important for application of the data to models. Borg and Grimes (1986) fitted time courses of maximum rooting depth of 48 crop species to a common parameterization by normalization of the data to relative time on a scale of days to attain maximum depth after seeding (DTM), and relative depth on a scale of the maximum root depth, RDm, where RD is relative depth and DAP is days after planting:
This gives a sigmoidal curve with an intermediate section that appears quasi-linear, a form that both our maximum and median curves appear to generally follow. The Borg and Grimes (1986) data set was taken from literature before any substantial use of minirhizotrons or higher quality, soil-recovered RLD was published, and much of it comes from trench-profile work. Modern minirhizotron data, with their more reliable representation of fine-root growth, provide more accurate median depth values than trench profile or root recovery techniques. Thus, it makes more sense to base parameterization of minirhizotron data primarily on more useful and indicative median depths, with maximum depth used as either a secondary parameter or a coparameter.
The sigmoidal type of parameterization such as that used by Borg and Grimes (1986) appears useful for time courses of both maximum and median rooting depths. Our data in Fig. 2 generally appears to fit sigmoidal parameterization. Except for dry pea data, the parts of the curves with steepest slopes project towards the x-axis to dates in May or June, dates near or clearly after seeding.
How do our results compare with those of others? The most available datum is maximum observed rooting depth. Merrill et al. (1994a) report a trench profile observation of safflower rooting to 1.9 m at the same general site as that used for the present study, which compares with the 1.8-m maximum depth observed in safflower in 1995 (the greatest depth that was possible to observe with our minirhizotrons). Our 1.4- to 1.6-m range of maximum sunflower depths compares favorably with a number of sunflower results in the literature. Jaafar et al. (1993) found a 2.2-m sunflower maximum with core-break technique in Kansas. Merrill et al. (1994a) reported a trench profile-observed maximum depth of 1.7 m on the same general site as the present study. Borg and Grimes (1986) quoted the classical trench profile observations of J.E. Weaver, who observed that sunflower rooted to depths of 1.5 to 3.0 m.
Our soybean maximum depths of 0.86 to 1.09 m compared with maximum depths of 1.15 to 1.45 m observed in three cultivars through use of recovered monoliths in southwestern Minnesota (Allmaras et al., 1975), 1.0 m observed with minirhizotrons, also in southwestern Minnesota (Nickel et al., 1995), and a 1.3-m maximum observed with minirhizotrons in Kansas (Grecu et al., 1988). Our dry pea maximum depths of 0.96 to 1.03 m compare with 0.8 to 1.0 m measured on green pea cultivars in Denmark with minirhizotrons by Thorup-Kristensen (1998).
There was a generally consistent pattern of maximum depth results, with six out of eight crops exhibiting their deepest maximum depths in the relatively wet year, 1995 (Table 4). Only crambe and dry pea had greatest maximum depths in relatively dry 1997. Results for greatest median depths (Table 4) showed no consistent pattern with years. Three crops had deepest medians in 1995, one in 1996, and four in 1997.
As reviewed by Zobel (1992), root depth and distribution are generally under the influence of many edaphic factors, but three in particular are influenced by management: nutrients, temperature, and water. Generally adequate nutrition may be assumed in a fertilized field experiment. Temperature influences root distributions in northern dryland crops, as soils heat up more slowly in the spring in wetter years, or in situations where management results in higher soil water content. In our study, the maximum root depths developed in late spring or summer, later than springtime warmup of soil. Thus, our observations of generally greatest maximum rooting depths occurring in 1995 are probably best explained as root growth responses to differences in soil water content that were created by the precipitation differences among the years of the study.
Two types of relationships between soil-water regime and root growth results are reported in the literature. In the first kind, comparing moderately watered situations with irrigated ones, or different degrees or patterns of irrigation, reports typically indicate that more shallow average rooting depth occurs under greater or more frequent irrigation regimes. Examples are Hoogenboom et al. (1987) comparing irrigated and nonirrigated soybean, and Merrill and Rawlins (1979) comparing different irrigation frequencies applied to sorghum [Sorghum bicolor (L.) Moench]. However, another type of water-regime relationship appears responsible for the general pattern of results reported here: typically in dryland situations, inadequate precipitation can limit rooting depth. Progressively greater subsoil drying during a 3-yr drought cycle caused the maximum rooting depths of spring wheat in a continuous crop rotation to decline from 1.10 to 0.75 m (Merrill et al., 1996). Our alternate crops were in continuous crop rotation, and our results showing lesser maximum root growth depths in six out of eight crops under average to below-average precipitation are most credibly explained as a response to lowered subsoil moisture.
Total Root Length Growth
Time courses of TRL are shown in Fig. 3
, with P-wall MR and Stand MR results distinguished. The time courses of many of crops reveal consistent patterns of root decay after midseason buildup of TRL. Cases where the data patterns measured with both types of minirhizotron consistently indicated that root decays had occurred, were all 3 yr for both safflower and sunflower, 1995 and 1996 crambe, and 1997 soybean. Another interesting result is the relatively higher peak TRL values measured in low-precipitation 1997 compared with 1995 and 1996 for the crops crambe, soybean, and dry bean.
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The pattern of our TRL results indicates a general fine-root growth response to lowered soil water availability. The TRL values averaged over minirhizotron type (Table 5) indicate a general increase in growth in drier 1997 compared with much wetter 1995. Five out of eight crops showed greatest TRL growth in 1997 (safflower, crambe, canola, soybean, and dry bean). Two crops had highest average TRL in 1996 (spring wheat and pea). A number of the crops showed considerable increases of average TRL in dry 1997 compared with wet 1995: 3.3 times higher for crambe, 2.5 times for dry bean, and 1.8 times for soybean. Only sunflower had highest average TRL growth in wet 1995. This is probably related to the fact that sunflower aboveground biomass yield (Table 2) was relatively even over the 3 yr, being highest in dry 1997.
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The average soil depths of the largest RLD profile values (GRLD, Table 3) were generally greater for P-wall MRs than for Stand MRs. In 1995, average depths where GRLD values were observed for P-wall MRs were greater than GRLD depths for Stand MRs in five out of eight crops; P-wall MR depths were greater than Stand MR depths in six out of eight crops in 1996; and P-wall MR depths were greater for all eight crops in 1997. The two greatest depths of rooting that occurred on each date for a given crop, and which were averaged to determine maximum rooting depth (Fig. 3), were observed in P-wall MRs more often than would be expected from the relative numbers of P-wall MRs vs. Stand MR's installed in each crop each year. These observed differences between the minirhizotron types are consistent with the interpretation that there were elevated soil strengths at wall-soil interfaces of Stand MRs, particularly deeper in the soil, and that this was a hinderance to root growth.
The amounts of TRL growth observed with the two minirhizotron types differed somewhat (Table 5). In 1995, the TRL for Stand MRs was greater than that for P-wall MRs for six out of eight crops, TRL for Stand MRs was greater than P-wall MRs for five out of eight crops in 1996, and Stand MR TRL was greater in six out of eight crops in 1997. This is consistent with the observation that Stand MRs were more optically efficient than P-wall MRs. Viewing is through two separate layers of material in P-wall MR's, as compared with only one layer in Stand MRs, and water condensation between inner and outer walls was a problem at lower depths for some P-wall MRs.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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For six out of the eight species, maximum depths of root growth were greater in relatively wet 1995. For a given species, differences in maximum rooting depth varied between years by about 0.1 to 0.3 m. However, with few and minor exceptions, the relative rankings of the crops for measures of rooting depth were the same over the 3 yr. These results increase confidence in the use of comparative-species root growth data and information collected in a given year for use in agro-ecological management.
The rooting depth results indicate considerable difference among the crops in potential for using water and nutrients at deeper positions in the soil profile. Average rooting depth values for first-ranked safflower (Table 4, maximum = 1.64 m, median = 0.92 m) were from 60 to 100% greater than those of last-ranked dry bean (maximum = 1.00 m, median = 0.46m). These rooting depth differences among the eight crops positively correlate with soil water extraction during the 1999 and 2000 growing seasons (May to October). Measurements on the same general site as the present study idicated that sunflowere and safflower had net depletions of soil water that were 20% to over 100% greater than those of the other six crop species (Merill et al., 2001a,b). These results show that inclusion of deeply rooted oilseed crops in dryland rotations can result in water and N use in subsoil positions, positively benefiting water quality in soil, land, and climate situations of excess precipitation or overland flow and unused, mobile fertilizer N.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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1 Mention of trade-names or products is for the convenience of the reader and does not indicate endorsement nor preferential treatment by the USDA-ARS. 
Received for publication January 10, 2001.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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values, which are defined here as the greatest quarter of values from root length density (RLD) profiles measured at the date of greatest total root length (TRL); average soil depth of GRLD values; and day of year (DOY) for greatest TRL. Values are tabulated separately for pressurized-wall and standard minirhizotrons.
